Load cells have been used in vehicle load weighing systems. Many designs of load cells have been used for detection of overload conditions in trucks for safety reasons. These systems were not used for customer billing because these systems did not have sufficient accuracy or precision. It was only necessary that the accuracy be great enough to detect an unsafe load level.
Known load cells utilize strain gauges which may include Wheatstone bridge arrays of fine wires which change resistance when under strain. Examples of related prior art are seen in U.S. Pat. Nos. 3,661,220 to Harris; 3,734,216 to Nordstrom et. al; 3,857,452 to Hartman; 4,020,911 to English et al.; 4,364,279 to Stern et al.; and 4,666,003 to Reichow.
Typically, prior art weighing systems used load cells that provided analog outputs to either central control boxes or to junction boxes where the analog signals representing weight would be converted to digital signals. Examples of these systems are U.S. Pat. Nos. 4,020,911 to English et al.; 4,456,084 to Miller. In a junction box system, the outputs of a number of load cells would be connected to a junction box which contains analog to digital converters, memory and a processor. The junction box would either sum the analog voltages representing weight and then convert the summed voltage representing total weight to a digital signal, or, alternately, convert the individual analog voltages to digital signals, and then sum the digital signals to obtain an overall weight. The digital signal indicating weight would then be transmitted to a system controller or display. One problem with junction box systems is that the analog signal that represents weight is subject to loss and noise between the load cell and the junction box. This limits the accuracy that can be achieved with these systems.
Some prior art load cells contained non-volatile memories to hold calibration data for the load cell. A microprocessor in the load cell would use the calibration data to adjust the gain and offset of the load cell, as in U.S. Pat. No. 5,710,716 to Hurst et al. The method of U.S. Pat. No. 5,814,771 to Oakes et al. is to calibrate a junction box on each trailer with the load cells on that trailer and store the calibration constants in a non-volatile memory in the junction box. The method of the Hurst et al. '716 Patent is to issue a command from the system controller 40 to the load cell to cause the load cell to enter a zero setting mode and a calibration setting mode. In the zero setting mode switches 32 apply a null voltage to amplifier 30. Analog to digital converter 26 reads the output voltage of amplifier 30 and micro-controller 20 determines an appropriate correction voltage to apply using a digital to analog converter connected to the non-inverting input of amplifier 30. A second zero is performed to zero the strain gauge when the vehicle is unloaded. A gain setting mode is entered into under control of the system controller when the truck has a known load, usually when it is on a scale. The gain setting mode determines a constant which is multiplied by the reading of the analog to digital converter to determine the load weight. No allowance for temperature changes is made. The limited error correction that these systems are capable of performing does not enable these systems to be of sufficient accuracy for customer billing.
U.S. Pat. No. 5,478,974 to O'Dea discloses an on-board vehicle weighing system that includes a “Tare” mode for display. The system, however, uses the pressure of air in the air suspension for the trailer and does not use high precision load cells. As such, the O'Dea system cannot achieve the accuracy of the present invention.
One prior art system, U.S. Pat. No. 5,789,714 to Doerksen, requires that the load be raised by hydraulic pistons in order to weigh it, and indicates that added time for the measurement to settle is required due to vibration caused by the lifting.
These prior art systems suffer from lack of accuracy and lack of resolution needed to maintain accurate customer billings. These systems are also susceptible to mechanical and electrical noise causing weight measurements to be degraded. Measurement drift due to temperature changes and creep of the strain gauge over time also degrades results.
Prior art on-board load weighing systems have not been certified or certifiable by weights and measures agencies because they were not sufficiently accurate for customer billing.